With the advent of high power gas lasers, the amount of power lost through inefficient operation is continuously increasing. This problem is important not only because of economic considerations, but also because the amount of energy being dissipated as heat has serious deleterious effects on the laser materials, resulting in degradation of its components. Maximum laser efficiency, can, in principle, be best achieved by tailoring the energy requirements to obtain excitation of the metastable state or the upper laser level. But unfortunately, this theory is generally inapplicable for existing gas lasers which employ conventional methods of gaseous discharge since in these cases, the average electron energy is largely dictated by the conditions and requirements for maintaining a sustained discharge.
For CO.sub.2 lasers involving vibration-rotation transitions, operation at high pressures is generally more desirable. By raising the gas pressure, the laser power output can conveniently be increased without increasing the cavity dimensions, thereby eliminating the need for using long cavities which are difficult to manage. Unfortunately, at high gas pressures, uniformly distributed electrical discharges are very difficult to maintain and laser oscillation may terminate because of arcing and localized current build up.
An example of arcing being caused by high gas pressures occurs in a pulsed CO.sub.2 laser where the high density of the active molecules requires that the energy per pulse per unit of volume increase linearly with the gas pressure. The increase in gas pressure becomes apparent, if it is recognized that population of the upper energy levels is generally induced by three methods. In the first method electrons collide directly with CO.sub.2 molecules. A second method involves an intermediate reaction and requires traces of a secondary gas, such as N.sub.2 within the discharge chamber. The electrons first collide with the N.sub.2 molecules and the excited N.sub.2 molecules, in turn, collide with ground state CO.sub.2 molecules. Each collision may add photons to the laser field, and thus the second method is often used in conjunction with the first to increase the power output of the laser. Without the presence of the secondary gas this intermediate reaction does not occur since after contributing photons to the laser field, the excited CO.sub.2 molecules are left in the lower state of laser transition. However, the molecules eventually decay to the ground state at which point they can be re-excited by collision with the N.sub.2 molecules. A third method may also increase the power output of the laser and involves deactivation of the lower laser levels by addition of a gas which depopulates the lower energy levels. The repitition rate of all the molecular collisions is directly related to the gas pressure and consequently, the maximum laser power available per unit volume depends on the operating pressure. Therefore, if high output power in a pulsed CO.sub.2 laser is desired, it is accompanied by an increase in pressure. This increased pressure can cause arcing since although the voltage drop across a cathode is essentially constant, a negative electrode within a plasma does not provide a uniform discharge distribution. Intense plasma distributed locally creates a bright or hot cathode spot having an energy input per unit area which increases rapidly with pressure to cause arcing. Also, the thickness of the cathode drop region decreases as the pressure increases causing the energy dissipated per unit volume in the cathode drop region to increase, resulting in current build up which, in turn, causes arcing. For all these reasons, the tendency of a glow discharge to change into an arc increases rapidly as the gas pressure is raised.
One solution to this problem has recently been proposed in an article titled "Transversely Excited Atmospheric Pressure Carbon Dioxide Laser" by A. J. Beaulieu. This article relates to a pulsed atmospheric pressure He--N.sub.2 --CO.sub.2 laser wherein a transversely excited discharge of short duration is maintained between the large flat anode and a row of resistive pin cathodes. A large number of these resistors are used to impede localized current build up which prevent local arcing of plasma and achieve a laser output of 2J per pulse at a repetition rate of 1,000 PPS and an efficiency of 17%. The current pulse for exciting the laser is obtained by discharging a 0.02 microfarad capacitor which had been charged to 17,000 volts. However, the discharge stability obtained was accompanied by a reduction of efficiency due to energy losses in the resistor network and therefore the Beaulieu device also has a considerable energy loss during its operation.